Flexible tubing and the manufacture thereof



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FLEXIBLE TUBING AND THE MANUFACTURE THEREOF Filed Feb. 1, 1965 2 Sheets-Sheet l Pg ciw w lNVENTOR LUDWIK JACHIMOWICZ AQW 3W ATTORNEYS.

THIS INVENTIQN PRIOR ART Sept 13, 1966 JACHIMOWICZ FLEXIBLE TUBING AND THE MANUFACTURE THEREOF Filed Feb. 1, 1965 2 Sheets-Sheet 2 l' INVENTOR LUDWIK JACHIMOWICZ' K m 5 0 a: a 5

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United States Patent 3,272,912 FLEXIBLE TUBING AND THE MANUFACTURE THEREOF Ludwik Jachimowicz, Elizabeth, N..l., assignor to General Cable Corporation, New York, N.Y., a corporation of New Jersey Filed Feb. 1, 1965, Ser. No. 429,526 19 Claims. (Cl. 174107) This invention relates to sheathed electrical cables of the type in which the sheath comprises a metallic strip which is coextensive in length with the cable core and is folded about the core into the form of a tube, and in which there is an outer jacket of polyethylene extruded over the tube. In such cables the longitudinal edges of the folded strip may overlap each other slightly, or they may substantially abut each other, or they may nearly abut each other with a slight gap between edges. More particularly the invention relates to such a construction in which the folded metallic strip loosely encloses the cable core, or is simply a duct or tube into which electrical wires and cables may be drawn subsequently, the folded metallic strip having insufiicient strength itself to withstand compression or collapse of the tube under the circumferential contraction during cooling of the polyethylene jacket extruded thereover in accordance with conventional practice. This invention provides an article and method of production in Which circumferential contraction and/ or collapse of the tube formed by the folded strip is prevented during contraction of the extruded polyethylene jacket as it cools, even though there is no mechanical connection between the edges of the strip, and no firm support inside the tube. a

It is an object of this invention to provide an improved non-collapsible cable and duct construction of the character described. It also is an object of this invention to provide an improved method of making non-collapsible cable and duct of the character described. Other objects and advantages of the invention will be set forth or will become apparent as the description proceeds.

This invention has been developed and will be described with particular reference to improvements in telephone cables, but the scope of the invention is to be determined by the claims. For further disclosure of the alpeth and stalpeth sheathed cables mentioned herein reference may be had to the paper, Bell System Cable Sheath Problems and Designs, by F. W. Horn and R. B. Ramsey, AIEE Proceedings, 1951, vol. 70.

The drawings forming a part hereof show preferred embodiments of the invention selected for illustrative purposes only. In the drawings:

FIGURE 1 is a diagrammatic top plan view of apparatus for making cable sheath or duct in accordance with the invention;

FIGURE 2 is a greatly enlarged sectional view taken on the line 22 of FIGURE 1;

FIGURE 3 is a greatly enlarged sectional view taken on the line 3-3 of FIGURE 1;

FIGURE 4 is a view similar to FIGURE 3, but showing a modified form of the invention;

FIGURE 5A is a transverse section through a cable in which the folded metallic strip loosely encloses the cable core with the strip edges overlapping, and showing the polyethylene jacket in place as the cable leaves the extruding machine, i.e., before the hot polyethylene has cooled and contracted;

FIGURE 5B is a view similar to FIGURE 5A showing what happens when the polyethylene jacket extruded in accordance with conventional practice cools and contracts;

FIGURE 6A is a view similar to FIGURE 5A, eX- cept that the folded metallic strip has fused to its outer 3 ,272,912 Patented Sept. 13, 1966 surface a thin layer of special material according to this invention;

FIGURE 63 is a view similar to FIGURE 6A showing what happens when the polyethylene jacket applied according to this invention cools and contracts;

FIGURE 7A is a view similar to FIGURE 6A, except that the longitudinal edges of the folded metallic strip are spaced slightly from each other in nearly abutting relation, instead of being overlapped;

FIGURE 7B is a view similar to FIGURE 7A showing What happens when the polyethylene jacket applied according to this invention cools and contracts; and

FIGURE 8 is a diagram for illustrating the principles involved.

Polyethylene jacketing compound (low density) expands by volume 26% when heated from room temperature of F. to extrusion temperature of 400 F. (R. D. Biggs and R. P. Guenther, Bell Telephone Laboratories, Eleventh Annual Wire and Cable Symposium, Asbury Park, New Jersey). Volume contraction between 400 F. and 80 F., taking the hot expanded volume as a reference base, is 21%. The effects of this shrinkage must be taken into account in the manufacture of cable having a jacket of polyethylene extruded thereon.

In the extrusion of polyethylene over a rigid supporting core such as lead sheathed cable, or cable having a soldered steel strip sheath, commonly known as stalpeth, the shrinkage of the extruded plastic as it cools produces only a reduction of the jacket wall thickness. Movement of the inner surface of the jacket concentrically towards the center of the cable during cooling is prevented by the firm support of the rigid lead sheath or the stalpeth steel sheath on which the jacket rests. The present invention makes it possible to prevent concentric contraction of the cooling plastic jacket in types of cables having a folded strip covering in which there is no rigid member under or over the folded strip to support the jacket against such contraction. Such types include the alpeth type sheath over an undersized or loose core, and empty duct or tubing,

If polyethylene is extruded as a hollow tube without support, or over a folded unsealed aluminum strip such as forms the tubular metallic wall in alpeth cable, the direction of shrinkage, by nature three dimensional, will be restrained only in the direction of cable length. It will be free in the radial and the circumferential directions. Assuming free movement, one half of the shrinkage will occur as radial shrinkage, i.e., reduction in wall thickness, and one half will occur as circumferential shrinkage, producing a reduction in the enclosed cross sectional area. To accommodate for 21% shrinkage by volume each of these two dimensions has to shrink linearly by about 10% The radial shrinkage, i.e., shrinkage in wall thickness, is not of particular concern in this case. The circumferential shrinkage, however, is of great importance because it will produce a reduction of the diameter of the jacket. If the plastic jacket is extruded without a support of any kind it collapses, as it cools, into an irregular shape of no utility.

To show the magnitude and effect of the radial shrinkage let us consider, for example, an alpeth cable having a diameter of 1.500" over the folded, overlapped aluminum strip, and a polyethylene jacket wall thickness at extrusion of 80 mils, i.e., 0.080". Ten percent (10%) shrinkage in wall thickness, that is radial shrinkage, will reduce the wall thickness by 10%, or by 8 m-ils. This may be taken into account in the thickness of the sheath as extruded. The same 10% reduction in circumferential direction will reduce the jacket circumference at the interface with the aluminum by 1500 1r 10%=450 mils, i.e., 0.450". The extruded jacket solidifies from the outside surface because the cable is passed from the extruder into cooling water. In alpeth cables the solidification of the outer shell of the extruded jacket presses the still molten inner stratum of the jacket inwardly towards the folded aluminum strip. The overlapped edges of the strip will slide relative to each otheruntil the aluminum strip is pressed tightly against the cable core. In this cooling process the outer overlapping edge of the strip may press into plastic jacket. This localized thinning of the plastic jacket is undesirable and can be overcome by use of a bridging tape, as disclosed in applicants Patent No. 3,087,007, April 23, 1963.

In alpeth cable the aluminum strip is normally wrapped around cable core rather snugly, allowing for sliding of the strip edges by only some 30 to 50 mils. After this the free movement of the aluminum strip under the pressure of the contracting plastic jacket has been exhausted, further circumferential shrinkage of jacket builds up the pressure from the jacket against cable core. This is shown very well on the inner surface of polyethylene jackets extruded over corrugated aluminum strips. The inner surface of the plastic is pressure molded into the corrugations of the aluminum strip. This pressure comes from the cooling and shrinking outer shell of the jacket which presses the plastic against the aluminum strip, now tfirmly supported on the cable core.

If there is no cable core, or if the core is soft, or too small to support the aluminum-strip, then the jacket extruded according to conventional practice continues to shrink circumferentially. As a result, the unsoldered aluminum strip collapses at the overlap. Soft jacket material flows into the collapsed region and a deformed cable or tube results with the folded aluminum strip rattling loose inside. No useful product is produced.

The present invention is concerned with those cases where it is desired to extrude a plastic jacket over a folded core-covering strip which is unfastened at the overlap and which encloses a soft or small core, or an empty circular cross section. This invention has utility, for example, in making a sheath over und-ulated cable core, or in making tubing for the flow of gas, or in making double sheathed cable when it is required that no indentations of the corrugated metallic strip be impressed into the underlying jacket, as in Minuteman cables. Until the present invention the only solution for these problems has been to fold the metallic strip so as to make the edges of the folded corrugated strip butted .up against each other and to warp or twist the strip intentionally to insure that the corrugations are out of registration so as to prevent one edge from sliding over the other. This undulated core cable construction is described in the Bell Laboratories Record, October 1964, pages 311- 3 12.

According to the present invention, the metallic strip which is folded into tulbular form about the cable core is coated, at least on its outer surface, with a thin film of special polyethylene compound which is intimately fused and firmly adhered on the surface of the metallic strip. This special compound, described also in applicants copending application Serial No. 320,777 now Patent No. 3,233,036, comprises polyethylene containing reactive carboxyl groups which have the ability to develop very firm adhesion to the aluminum strip and also to the polyethylene jacket when it is extruded thereover. Aluminum strip coated with this special compound is sometimes known as fused polyethylene aluminum strip.

When the polyethylene jacket compound is extruded onto the tube formed by a folded strip of fused polyethylene aluminum the hot plastic jacket adheres instantly. and firmly to the thin film of special polyethylene compound on the outer surface of the folded strip. Applicant has discovered that this adherence limits the freedom of movement for the material of the jacket as it cools. Shrinkage of the cooling jacket is restrained in all directions but one. The jacket can shrink only in the direction of wall thickness.

Referring to the drawings, FIGURE 1 shows a loose cable core 10 which is advanced with uniform motion in the direction indicated by the arrows 12. A coated metallic strip 14 is advanced with the cable core 10 and is folded about the cable core 10 by pinch rollers 16. These pinch rollers 16 are merely representative of forming apparatus for bending the strip 14 into a tubular shape'with the opposite edges 18 of the strip abutting or substantially abutting, as illustrated in FIGURE 3, or overlapping, as illustrated in FIGURE 4, along a longitudinal seam 20. To simplify the illustration only one pair of pinch rollers 16 is illustrated, but it will be understood that additional forming rollers can be employed in accordance with conventional tube forming practice.

The coated metallic strip 14 includes a transversely corrugated aluminum strip 22. The corrugations are clearly shown in FIGURE 2. There is a thin plastic coating 24 on the side of the metallic strip 22 which forms the outside surface of the tube after the strip has been folded about the core 10. Preferably, there is a similar coating 26 on the other side of the strip 22 which forms the inside of the tube. This inner coating is not essential to this invention, but it provides important protection for the metallic strip against corrosion. The coatings 24 and 26 are applied to the metallic strip 22 before it enters the apparatus illustrated in FIGURE 1, and the laminated strip 14 may be stored on a reel (not shown) from which it is withdrawn for the forming operation illustrated in FIGURE 1.

The plastic coatings 24 and 26 are polyolefin plastic combined with material to produce an adhesive composition which will form a firm bond to the metallic strip 22 and to which an extruded plastic will adhere instantly upon contact. The coatings 24 and 26 should be flexible protective films which have high electrical resistivity, high resistance to chemicals and moisture, and exceptionally .good adhesion to the aluminum strip 22 to withstand manufacturing processes such as the corrugating of the laminated strip 14, and to prevent delamination in corrosive atmospheres or liquids.

Polyethylene films generally satisfy the requirements of resistivity and resistance to chemicals and moisture. However, polyethylene does not adhere to metals with the desired bond because the polyethylene is inert and can develop only a mechanical bond based on a friction-type adhesion. Best results are obtained with a copolymer of polyethylene and acrylic groups, for example, a graft copolymer of polyethylene and a monomer with a reactive carboxyl group such as an acrylic acid or an acrylic acid ester, as described in my copending application Serial No. 320,777, now Patent No. 3,233,036, or in U.S. Patents Nos. 3,987,501 and 3,027,346. The carboxyl component of the copolymer has the property of forming chemical bonds with the metals to provide the desired bonding of the film to the metal.

The nature of the copolymer is such that when it is applied on both sides of the aluminum strip 22 in a thickness ranging from one to three mils it will prevent direct attack on the metal surface when exposed to corrosive environments anticipated in the installation of telephone cable in varied locations. If the film coatings 24 and 26 are damaged at any point the metal may be exposed to corrosive attack at the point of damage, but the rate of degradation of the metallic strip 22 is very much slower than it would be for an uncoated metallic strip, because the corrosive action enlarging the area of the metal destroyed must proceed along the length of thestrip between the protective coatings 24 and 26 In order to so define any path of corrosion the coatings 24 and 26 must adhere to the metallic strip without delamination under the exposure to corrosive conditions and the mechanical forces of the corrosion products.

Commercially available materials suitable for the coatings 24 and 26 are obtainable from the Dow Chemical Company of Midland, Michigan, under the designations Copolymer Resin QX-3623 and Copolymer Resin QX-4262.6.

By way of illustration, a strip of aluminum 8 mils thick was coated on both sides with 2 mils of the special polyethylene compound according to this invention. The coated strip was corrugated transversely, the corrugation depth being 50 mils, with 9 to 10 corrugations per inch. The corrugations are not essential to the present invention, but provide flexibility for the sheathed cable or duct. The thickness of the metallic strip and the thickness of the strip coatings are exaggerated in FIGURES 2 through 4 and 6A through 7B, this being necessary to show the construction clearly.

Beyond the pinch rollers 16, the core with the folded tubular covering formed by the folded strip 14 passes through an extrusion jacketing station 30 wherean extruder 32 extrudes plastic 34, preferably polyethylene, by the sleeving process over the outside surface of the folded strip 14 to form a jacket 36. This outer jacket 36 preferably is made of black polyethylene of the kind com monly used for the outside covering of electrical cables. At the temperature at which the plastic 34 is extruded it adheres instantly on contact with the material of the outer coating 24 and produces a laminated construction with the coated aluminum strip 14. The outer jacket 36 is preferably of a radial thickness between 60 and 80 mils.

The folded strip 14 is not sticky as it enters the extruder 32, but becomes activated by the heat of the jacket when it comes into contact with the jacketing material. Once adhesion of sufficient strength is developed between the jacket 36 and the folded strip 14, the shrinkage of the jacket 36 is restricted in all directions but one, that is, in direction of the wall thickness. The adhesive should be of such nature as to become stronger as it cools, because contraction forces in the jacket also increase as the jacket cools, from a few pounds per square inch when hot to about 1800 pounds per square inch when at room temperature.

Adhesion of the outer jacket 36 to the folded strip depends on the temperature at which the plastic 34 is extruded over the folded strip 14. It is desirable to keep the temperature of the plastic 34 within the range of 400-450" F. for high molecular weight polyethylene and for extrusion speeds of fifty feet per minute and up. This is compatible with normal manufacturing processes.

FIGURE 3 shows the coated metallic strip 14 formed around a loose cable core 10, and the cable core is not sufficiently large to provide any support for the folded strip 14 against circumferential contraction and collapse. In order to prevent sagging of the strip covered core a supporting roll 46 can be used along the run between the forming station or pinch rollers 16 and the extrusion and jacketing station 30.

FIGURE 4 shows a modified construction in which the coated metallic strip 54 is formed around the cable core 10 with a lapped seam 56 instead of the butt seam of FIGURE 3. All other parts in FIGURE 4, which correspond to those in FIGURE 3, are indicated by the same reference characters, but with a prime appended.

FIGURES 5A and 5B illustrate what happens when a jacket of polyethylene is extruded onto the overlapping folded aluminum strip covering a loose cable core, in accordance with conventional practice. FIGURES 6A and 6B show, by way of direct comparison, what happens when a jacket of polyethylene is extruded onto the overlapping folded metallic strip covering over a loose cable core, in accordance with the present invention. The constructions and procedures of the tests on which these illustration-s are based were the same in both cases except that in FIGURES 5A and 5B the aluminum strip was uncoated, whereas in FIGURES 16A and 6B the aluminum strip was coated with the special adhesive polyethylene compound as described hereinabove.

In FIGURE 5A the cable core 61 is shown loosely enclosed within the approximately cylindrical tube formed from the folded aluminum strip 62. The strip edge overlapped each other, as shown. The cylindrical sleeved jacket 63 of polyethylene is shown as it came hot from the extruder. As the polyethylene jacket 63 cooled it contracted circumferentially, causing the overlapping edges of the aluminum strip to slide, one with respect to the other, thus reducing the cross sectional area enclosed by the folded strip and distorting the original substantially cylindrical shape. The outermost edge of the strip gouged into the polyethylene jacket and made it thinner directly over the strip edge than elsewhere around the periphery. This result is shown in FIGURE 5B.

In FIGURE 6A the cable core 61 is shown loosely enclosed within the approximately cylindrical tube formed by the folded aluminum strip 62', the strip edges overlapping each other as shown. The sleeved jacket 63 of polyethylene, still hot from the extruder, was cylindrical. The heat of the jacket activated the special polyethylene coating on the outer surface of the strip 62, resulting in instant adhesion of the jacketing material to the coated strip around the periphery thereof. After the polyethylene jacket 63 cooled it contracted, but the strip 62' was not free to slide circumferentially under the jacket 63' and shrinkage of the jacket was restricted to radial shrinkage, resulting only in a reduction in the thickness of the jacket Wall. There was no sliding of the overlapped edges of the folded strip relative to each other and the resulting construction was as shown in FIGURE 6B.

The construction shown in FIGURE 7A was similar to that in FIGURE 6A, except only that instead of the folded strip edges overlapping each other there was a slight gap between them. FIGURE 7A shows the construction with the polyethylene jacket in cylindrical form, still hot, as sleeved onto the folded strip 62" of aluminum having on its outer surface a coating of the special adhesive compound, in accordance with this invention. Instant adherence of the hot jacket 63" to the folded strip 62" prevented movement of the jacket relative to the surface of the strip. As the material of the jacket 63 cooled it was restricted to contraction in a radial direction except only for a very narrow circumferential area over the gap between the edges of the folded strip. The contraction resulted in a thinning of the jacket 63" except at the slight gap between the edges of the folded strip, where the compound did enter slightly and bulge down into the gap, as shown in FIGURE 7B.

In order to prevent circumferential shrinkage of the thermoplastic jacket as it coo-ls the inner surface of the jacket must retain a constant circumferential length in spite of the shrinkage of the mass of the overlying jacketing material. In conventional alpeth cable there is no adhesion of the polyethylene jacket to the aluminum and the cooling of the extruded jacket causes the aluminum tube to col-lapse partially until it is supported by the cable core. According to the present invention the polyethylene jacket adheres firmly to the special adhesive polyethylene coating on the metallic strip at the time of extrusion and there is no collapsing of the metallic tube.

Reference is made to the diagram of FIGURE 8 for an explanation of the principles involved. Consider a segment of the thermoplastic jacket and the metallic strip. If the jacket 1 is made to adhere to the metallic strip 2 at points A and B, then as the jacketing material shrinks the metallic strip 2 will be subjected to compressing force S. Compressing force S develops as a result of the jacket shrinkage and varies with extrusion conditions. In prevailing cases it may reach a level of some 500 p.s.i. The metallic strip 2 could be regarded as a column under stress if points A and B are close enough together. For stresses in a compressed aluminum column the following formula is given in the handbook, Aluminum Structural 7 Design, by Reynolds Metal Company, Louisville 1, Kentucky, 1951:

P 1012 6,200 p.s.1.

where P/a=ultimate strength of column in p.s.i. k=coefficient of end fixing L=length of the column, and

r'=radius of gyration of the strip If the jacket has thickness 0.080", which is typical for telephone cables, the force per 1" of axial length of cable is 500 X (1.000 0.080)=40 lbs.

Stress S imposed onto the aluminum strip at points A and B is If adhesion of the jacket to the supporting strip is nearly continuous, then the distance L between points A and B approaches zero and the aluminum strip will not collapse, because P/a=6,2.00 5000 p.s.i.

If, however, adhesion is spotty and the distance L reaches 6200-5000=7750L, or L=0.l55",

the strip will collapse and the jacket will shrink circumferentially.

The actual mechanics of preventing circumferential shrinkage of the jacket is more complex. At first the force S is small, because the jacket is in a molten state. So is the bond between the jacket and the molten coating of the strip at points A and B. As the cable cools the force S increases, requiring stronger bonds at points A and B. This is provided by the copolymer coating of the strip which, being also polyethylene, cools and acquires strength in the same temperature bracket as the jacketing material. Gradually then, as the force in the cooling jacket increases, the bonding to the metal increases, and with it the ability to convey the forces to the metal. At ultimate cooling, stresses in the jacket are conveyed to the aluminum strip which supports them without collapsing or wrinkling. Experiments conducted at various extrusion conditions have shown that the borderline of thickness of aluminum strip which can be used without collapsing is 4 mils. Aluminum strip at 8-mil thickness provides a safety margin even when the adhesion between the jacket and the strip is less than continuous. The metallic strip folded to form the tubular covering for the cable core has been described in certain of the illustrations as being aluminum, but it might be made of copper or other material.

A preferred embodiment of the invention has been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without departing from the invention as defined in the claims.

I claim:

1. A jacketed electrical cable comprising a core of insulated conductors, a metallic strip coextensive with =5000 p.s.i.

the core and folded into tubular form thereabout, and an extruded jacket of plastic enclosing the tubular form, characterized by the fact that the said tubular form loosely encloses the core unsupported thereby, and by the further fact that the plastic is a thermoplastic material exhibiting a volume contraction of about 20% between extruding temperature and atmospheric temperature, and by the further fact that the plastic jacket is firmly bonded to the metallic strip at the interface so as to prevent relative movement between the jacket and the folded strip at the interface and thereby prevent collapse of the otherwise unsupported tubular form.

2. A cable according to claim 1 in which there is a thin layer of plastic highly adhesive to metal firmly bonded to the outside surface of the metallic strip and in which the relatively thicker overlying plastic jacket is bonded to the thin layer of adhesive plastic.

3. ,A cable according to claim 2 characterized by the thin layer of adhesive plastic on the metallic strip being a copolymer of ethylene and a monomer containing reactive carboxyl group.

4. A flexible duct comprising a metal strip of indefinite length folded into tubular form with the strip edges free for relative movement, a thin layer of a highly adhesive plastic firmly bonded to the outer surface of the strip, and a relatively thicker jacket of plastic having a high coeflicient'of expansion enclosing and firmly bonded to the thin layer of adhesive plastic so as to prevent relative movement between the outer plastic jacket and the folded strip and between the strip edges.

5. The flexible duct described in claim 4 characterized by the strip being corrugated with transversely extending corrugations throughout the full width of the strip.

6. The flexible duct described in claim 4 characterized by the strip being aluminum and the thicker outer coating being made of polyethylene.

7. The flexible duct described in claim 4 characterized by the layer of adhesive plastic on the strip being a graft copolymer of polyethylene and a monomer with a reactive carboxyl group.

8. The flexible duct described in claim 7 characterized by the monomer with the reactive carboxyl group being an acrylic acid.

9. The flexible duct described in claim 6 characterized by the coating on the strip being a graft copolymer of polyethylene and a monomer with a reactive carboxyl group including an acrylic acid ester, the aluminum strip being between five to ten mils thick and the coating on the aluminum strip being from one to three mils thick.

10. The flexible duct described in claim 4 characterized by the strip being aluminum approximately eight mils in thickness and coated with adhesive plastic to a thickness of approximately two mils, and the strip being corrugated transversely with corrugations having a depth of approximately fifty mils and about five to ten corrugations per inch of length of the sheet.

11. The flexible duct described in claim 4 characterized by the cross section of the tube being substantially circular and the longitudinal edges of the sheet being in substantial abutment with one another.

12. The flexible duct described in claim 4 characterized by the cross section of the tube' being substantially circular and the longitudinal edge portions of the tube overlapping one another.

13. The method of making a plastic jacketed metallic duct of preselected tubular cross section and size and in which the plastic jacketing material, having a high coeflicient of expansion, is extruded hot onto the preformed metallic duct, which method comprises moving a metallic strip in the direction of its longitudinal axis, continuously folding the strip into the preselected tubular cross section and size'while leaving the adjacent longitudinal edges of the folded strip free to move relative to each other, extruding the jacketing material hot onto the tubular form, and preventing circumferential contraction of the tubular form as the jacketing material cools by causing the hot jacketing material to adhere firmly to the tubular form around its periphery at the instant of contact and by such adherence to reinforce the tubular form against collapse.

14. The method of claim 13 in which the jacketing material is polyethylene and in which the method includes the step of bonding to the surface of the strip which will become the outside of the tubular form a thin layer of polyethylene containing reactive carboxyl groups.

15. The method of claim 13 which includes folding the strip into preselected tubular form with the strip edges overlapping each other.

16. The method of claim 13 which includes folding the strip into preselected tubular form with the strip edges spaced slightly from each other.

17. The method of making a plastic jacketed duct which comprises bonding to one surface of a strip of indefinite length a thin coating of a firmly adhering plastic material, moving the strip longitudinally and continuously folding the strip into tubular form with the plastic coating on the outside of the tubular form and with the edges of the strip free to move relative to each other, and sleeving a tube of plastic jacketing compound having a high coefiicient of expansion onto the tubular form at a temperature which will activate the coating on the folded strip to produce instant bonding between the said coating and the jacketing materialat the interface to reinforce the folded strip against collapse and prevent circumferential contraction of the jacket and relative movement between the strip edges.

18. The method of making a plastic jacketed electrical cable which comprises bonding to one surface of a metallic strip a thin coating of a firmly adhering plastic material, drawing the strip along a moving conductor core, continuously folding the strip into tubular form about the moving core with the plastic coating on the outside of the tubular form while leaving the longitudinal edges of the strip free to move relative to each other, and sleeving a tube of plastic jacketing compound having a high coefiicient of expansion onto the tubular form at a temperature which will activate the coating on the folded strip to produce instant bonding between the said coating and the jacketing material at the interface to reinforce the folded strip against collapse and prevent circumferential contraction of the jacket and relative movement between the strip edges.

19. The flexible duct described in claim 4 characterized by the strip being aluminum at least four mils thick.

References Cited by the Examiner UNITED STATES PATENTS 1,702,332 2/1929 Apt 174-107 2,987,501 6/1961 Rieke et a1. 260878 X 3,027,346 3/1962 Rugg et al. 260878 3,130,256 4/1964 Mildner 174107 X LEWIS H. MYERS, Primary Examiner.

JOHN F. BURNS, Examiner.

D. KETTLESTRINGS, Assistant Examiner. 

1. A JACKETED ELECTRICAL CABLE COMPRISING A CORE OF INSULATED CONDUCTORS, A METALLIC STRIP CO-EXTENSIVE WITH THE CORE AND FOLDED INTO TUBULAR FORM THEREABOUT, AND AN EXTRUDED JACKET OF PLASTIC ENCLOSING THE TUBULAR FORM, CHARACTERIZED BY THE FACT THAT THE SAID TUBULAR FORM LOOSELY ENCLOSES THE CORE UNSUPPORTED THEREBY, AND BY THE FURTHER FACT THAT THE PLASTIC IS A THERMOPLASTIC MATERIAL EXHIBITING A VOLUME CONTRACTION OF ABOUT 20% BE- 